The operation of electronic circuits, such as those within computers, telecommunications devices, and the like, causes electromagnetic radiation. Electromagnetic radiation consists of electromagnetic waves that propagate through the open atmosphere. Electromagnetic radiation from the operation of electronic circuits is undesirable because it can interfere with the performance of electronic systems, and can also interfere with radio broadcasts, cellular telephone signals, and other radio signals.
Electromagnetic (EM) radiation from electronic circuits, however, only causes undesirable interference to the extent that the EM energy radiates externally from the device containing the circuit. Accordingly, to control interference with communication signals, efforts are undertaken to inhibit the radiation of EM energy external to such devices. In fact, governmental agencies regulate the amount of external electromagnetic (EM) radiation that may be generated by such devices.
As a result, one element of product design for devices that include electronic circuits includes controlling external EM radiation, or in other words, containing the EM energy generated by the electronic circuit. One method of containing EM radiation is to provide EM shielding on the device. EM shielding typically consists of electrically conductive material that is grounded and acts as a barrier for the EM energy generated by the circuit. Proper shielding can substantially reduce the amount of harmful EM energy that is externally radiated by an electronic circuit. However, EM shielding increases the material and labor cost of products, and often dictates physical attributes and dimensions of the product.
In particular, it is noted that one of the most effective EM shielding arrangements consists of a complete skin of grounded material that completely encloses the electronic circuit. It has also been observed that a circuit may also be well-shielded by a grounded enclosure that has openings, as long as no opening has a dimension as large or larger than one-fourth the wavelength associated with the highest effective operating frequency of the circuit. Such a grounded enclosure is often referred to in the art as a Faraday cage. The phrase "effective operating frequency", as used herein, refers to a frequency of problematic radiation generated by the operation of the circuit. The effective operating frequency may be induced by the clock speed, for example, and may be a fraction or a multiple thereof.
Apart from EM radiation control, there are other constraints in electronic circuit design that can require an electronic circuit to be enclosed. For example, a circuit may require liquid cooling, and thus require a fluid tight enclosure. Yet other circuits may require an enclosure to provide physical isolation from a harsh operating environment. Still other circuits may require an enclosure to segregate heat tolerant electronic components from heat sensitive electronic components or vice versa.
It is noted, however, that enclosures required for such other constraints have different requirements than enclosures utilized for EM shielding. In particular, physical enclosures require some facility for placing the circuit, which is usually affixed to one or more circuit carry substrates, for example a printed circuit board, into the enclosure. In order for the enclosure to allow placement of the circuit into the enclosure, it must typically have a significant opening. The opening provides a path through which electromagnetic radiation may escape. Thus, enclosures required for physical constraints often are inadequate to also serve as the EM shielding of a circuit, particularly for circuits radiating high frequencies in which the wavelength is on the order of centimeters or less.
One prior art method of enclosing a circuit and providing EM shielding included placing the circuit within a grounded enclosure with a door. Surrounding the door are a plurality of spaced apart springs that provide electrical contact between the door and rest of the enclosure. The springs are electrically conductive and elastically deformable. The springs are spaced to prevent emission of radiation at wavelengths of interest. Springs are used instead of other structures because the elasticity of the springs adjusts to accommodate uneven spacing tolerances between the enclosure and the door, thereby ensuring that each spring is in electrical contact with both the door and the enclosure.
A problem associated with the above described enclosure is the labor and material cost associated with the use of springs. The spring material itself adds significantly to the cost of the enclosure. Moreover, the labor required to affix the springs to the enclosure, which is typically done by hand, is substantial. Furthermore, the springs are delicate and prone to damage.
Accordingly, there is a need for an arrangement of electromagnetically shielding an electronic circuit that incorporates a circuit enclosure that avoids the manufacturing costs associated with the use of an enclosure having spaced apart springs. There is a further need for an arrangement for electromagnetically shielding an electronic circuit that incorporates a circuit enclosure that fulfills a separate physical isolation requirement of the electronic circuit. There is further a need for a method of manufacturing such a device that is cost-effective.